Pilot images, which are commonly used in CT technology, are mainly used to roughly reflect anatomical structures of human organs and positions of large lesions, so as to assist doctors to scan and position. Commonly used scanning methods include single pilot image scanning or double pilot images scanning. Referring to FIG. 1, a schematic diagram of pilot image scanning is illustrated, which includes a CT gantry (1), an X-ray tube (2), a detector (3), a patient table (4), and a scanning object (5). In the process of pilot image scanning, the CT gantry is fixed, which means the X-ray tube and the detector are fixed, and the patient table which carries the scanning object moves forward or backward at a constant speed to obtain scanning data. The double pilot images scanning is similar to the single pilot image scanning. After scanning an anteroposterior pilot image of the scanning object (if the scanning object is a human body, scanning from the front side of the human body, which is perpendicular to the plane of the patient table in FIG. 1), a lateral pilot image is scanned (scanning from the lateral side of the human body, that is the arm side of the human body in FIG. 1). With a simple imaging principle and a fast imaging speed, the above mentioned scanning methods can basically meet the doctors' needs for scanning and positioning. However, geometric distortion exists in pilot images obtained by the above mentioned scanning methods.